In recent years, a significant amount of research and development effort has focused on improving methods for preparing polymeric materials and on polymeric materials having improved laser ablation properties.
Illustratively, cationic UV curable systems have received considerable attention and are experiencing rapid development due to advantages, such as lack of oxygen inhibition, low energy requirement, a solvent free process, and high polymerization rate (Lazauskaite et al., “Cationic Photoinduced Polymerization. Photoinitiators and Monomers,” Environmental and Chemical Physics, 24(2):98-117 (2002) (“Lazauskaite”); and Sangermano et al., “Coatings Obtained Through Cationic UV Curing of Epoxide Systems in the Presence of Epoxy Functionalized Polybutadiene,” J. Mater. Sci., 37(22):4753-4757 (2002), which are hereby incorporated by reference). Among the many monomer-photoinitiator combinations for cationic UV curable formulations, the combination of cycloaliphatic epoxide monomer and onium salt (especially sulfonium salt photoinitiators) have been widely explored since these are believed to provide, for example, higher reactivity, higher thermal stability, and good coating properties (Lazauskaite; Crivello et al., “Preparation and Cationic Photopolymerization of Organic-Inorganic Hybrid Matrixes,” Chem. Mater., 9(7):1562-1569 (1997); and Crivello, “Photoinitiated Cationic Polymerization,” Ann. Rev. Mater. Sci., 13:173-190 (1983) (“Crivello I”), which are hereby incorporated by reference). However, the curing speed and reactive functional group conversion for these cationic UV curable systems are still not as high as free radical systems (Lazauskaite, which is hereby incorporated by reference). It is believed that one of the important reasons for this is that the onium salt photoinitiators have a confined absorption in the long wavelength region of the UV spectrum. Thus, much of the energy emitted by broadband light sources, such as the commonly used mercury arc lamps, is wasted (Crivello et al., “Anthracene Electron-Transfer Photosensitizers for Onium Salt Induced Cationic Photopolymerizations,” Journal of Photochemistry and Photobiology A: Chemistry, 159(2):173-188 (2003) (“Crivello II”), which is hereby incorporated by reference). To overcome this drawback, the addition of photosensitizers may be used to extend the sensitivity of the coating system at longer UV wavelengths (Crivello et al., “Visible and Long-Wavelength Photoinitiated Cationic Polymerization,” Journal of Polymer Science: Part A: Polymer Chemistry, 39(3):343-356 (2001); Gomurashvili et al., “Phenothiazine Photosensitizers for Onium Salt Photoinitiated Cationic Polymerization,” Journal of Polymer Science, Part A: Polymer Chemistry, 39(8):1187-1197 (2001); and Sangermano et al., “Visible and Long-Wavelength Cationic Photopolymerization,” pp. 242-252 in Belfield et al., eds., Photoinitiated Polymerization, No. 847, ACS Symposium Series, New York: Oxford University Press (2003), which are hereby incorporated by reference). Electron transfer photosensitization is considered the most efficient and generally applicable process for onium salt photoinitiators, which is in essence a photoinduced redox process (Crivello II; Pappas et al., “Photoinitiation of Cationic Polymerization. III. Photosensitization of Diphenyliodonium and Triphenylsulfonium Salts,” Journal of Polymer Science: Polymer Chemistry Ed., 22(1):77-84 (1984); and Fatmanur et al., “Photosensitized Cationic Polymerization of Cyclohexene Oxide Using a Phenacylanilinium Salt,” Macromolecular Rapid Communications, 23(9):567-570 (2002) (“Fatmanur”), which are hereby incorporated by reference). A generalized photosensitization mechanism for onium salts is shown in Scheme 1 (Crivello II, which is hereby incorporated by reference) using a diaryliodonium salt as an example; similar mechanisms can be written for the photosensitization of other onium salt photoinitiators such as triarylsulfonium and dialkylphenacylsulfonium salts.

As illustrated in Scheme 1, the photosensitizer (“PS”) is first raised to the excited state after the absorption of light (eq. 1); then, an excited state complex (exciplex) is formed as an intermediate between the onium salt and the excited photosensitizer (eq. 2). Subsequently, an electron is transferred from the PS to the photoinitiator, which induces its decomposition and yields a diaryliodine free radical and the photosensitizer cation radical paired with the anion MtXn− (eq. 3). The rapid decomposition of the resulting unstable diaryliodine free radical (eq. 4) prevents the back electron transfer and renders the overall process essentially irreversible. The cationic polymerization (eq. 5) takes place either by the direct interaction of the monomer with the photosensitizer cation radical or by first radical dimerization and then polymerization by the resulting dication. During electron transfer photosensitization, both the onium salt and the photosensitizer are irreversibly consumed (Crivello II; and Crivello et al., “Curcumin: a Naturally Occurring Long-Wavelength Photosensitizer for Diaryliodonium Salts,” Journal of Polymer Science: Part A: Polymer Chemistry, 43(21):5217-5231 (2005), which are hereby incorporated by reference). Due to their higher reduction potential, sulfonium salt photoinitiators, unlike diaryliodonium salts, are most effectively sensitized by electron rich polynuclear aromatic compounds such as anthracene, pyrene and perylene (Fatmanur; Hua et al., “Photosensitized Onium-Salt-Induced Cationic Polymerization with Hydroxymethylated Polynuclear Aromatic Hydrocarbons,” Chem. Mater., 14(5):2369-2377 (2002) (“Hua”); and Pappas et al., “Cationic UV curing,” Advances in Organic Coatings Science and Technology Series, 4 (Int. Conf. Org. Coat. Sci. Technol., Proc., 6th, 1980):103-107 (1982), which are hereby incorporated by reference). However, such electron rich polynuclear aromatic compounds have properties and toxicities that can render them unsuitable or otherwise limit their effectiveness as photosensitizers in the coating matrix. For this and other reasons, new electron rich polynuclear aromatic photosensitizers are needed, and the present invention, in part, is directed to addressing this need.
As noted above, polymeric materials having improved laser ablation properties continues to be a focus of much research and development effort. Laser ablation of polymeric materials is receiving more and more attention due to its advantages in many potential applications, such as the fabrication of microfluidic devices (Pugmire et al., “Surface Characterization of Laser-Ablated Polymers Used for Microfluidics,” Anal. Chem., 74(4):871-878 (2002), which is hereby incorporated by reference) and microelectronic/optical parts (Kunz et al., “Photoablation and Microstructuring of Polyestercarbonates and Their Blends with a XeCl Excimer Laser,” Appl. Phys. A., 67(3):347-352 (1998) (“Kunz”), which is hereby incorporated by reference), polymer fuel in laser plasma thrusters (Lippert et al., “Fundamentals and Applications of Polymers Designed for Laser Ablation,” Appl. Phys. A., 77(2):259-264 (2003) (“Lippert I”), which is hereby incorporated by reference), and the like. The mechanism of laser ablation involves both pyrolysis (thermal decomposition) and photolysis (photochemical decomposition) of the material (Lippert, “Laser Application of Polymers,” Adv. Polym. Sci., 168:51-246 (2004), which is hereby incorporated by reference). Photolysis is the preferred mechanism in terms of ablation resolution since the involvement of thermal processes can lead to unwanted deviation from the optimum quality of the structure (Kruger et al., “Ultrashort Pulse Laser Interaction with Dielectrics and Polymers,” Adv. Polym. Sci., 168:247-289 (2004) (“Kruger I”), which is hereby incorporated by reference). Cleaner, higher resolution laser ablation is made possible by advances in laser technology and novel material development. The use of an ultrashort pulse laser such as a femtosecond laser provides a much higher ablation resolution due to the minimization of laser induced heat effects (Kruger I; Kruger et al., “Femtosecond-Pulse Visible Laser Processing of Transparent Materials,” Applied Surface Science, 96-98:430-438 (1996); and Serafetinides et al., “Ultra-Short Pulsed Laser Ablation of Polymers,” Applied Surface Science, 180 (1-2):42-56 (2001), which are hereby incorporated by reference). On the material side, new photopolymers have been designed and synthesized. For example, polymers containing the triazene group in the backbone have been synthesized. The photosensitive triazene group absorbs the incident laser energy and photochemically decomposes into gaseous nitrogen, causing the fracture of the polymer backbone; following which, the nitrogen generated (called “carrier gas”) ejects out of the ablation site with supersonic velocity, carrying away polymer fragments and ablation debris, resulting in a cleaner, higher resolution ablation structure (Lippert I; and Nuyken et al., “Excimer Laser Ablation of Triazene-Containing Polyesters with Different Topologies,” Acta Polym., 49(8):427-432 (1998) (“Nuyken”), which are hereby incorporated by reference). Polyestercarbonates have also been synthesized and ablated by a 308 nm Excimer laser. Because of their absorption at the incident laser wavelength and the gaseous photochemical decomposition products of ester group such as CO and CO2, they were reported to be ablated faster with a higher resolution of the ablated microstructure (Kunz, which is hereby incorporated by reference).
Cycloaliphatic epoxide based cationic UV curable coatings can be advantageous candidates for microelectronic packaging materials, for example, due to good electrical properties (Koleske et al., “UV-Cured Cycloaliphatic Epoxide Coatings,” National SAMPE Technical Conference, 14 (Mater. Process Adv. 1982):249-256 (1982); and Koleske et al., “Technology of Cationic, UV-Cured Cycloaliphatic Epoxides,” National SAMPE Technical Conference, 16 (Hi-Tech. Rev. 1984):529-536 (1984), which are hereby incorporated by reference) and low shrinkage rate during UV curing. Using a 355 nm laser to ablate through vias in such coatings is one of the steps in a specific microelectronic fabrication process, but few reports can be found on either 355 nm laser ablation of polymers or laser ablation of UV curable materials. Previously in this lab the 355 nm laser ablation performance of cationic UV curable coatings was successfully improved by incorporating ˜1 wt % of reactive laser ablation sensitizers as an additive. However, a need continues to exist for methods and materials which can be used to make polymers that have improved laser ablation properties, and the present invention is directed, in part, to addressing this need.